EditorTree node widgets created from HTML have inconsistent state

Description

If I create an EditorTree? and some nodes all using HTML, all nodes
are visible, but internally the top-level nodes are set up as not
expanded, i.e. the expander icons show them as not expanded, first clicks
do not close them, all this regardless of initial value of the isExpanded
property.

Setting up the initial value of containerNode.style.display='none'
solves all of these problems at once. I did it in the templateString,
but some other way might be preferred.